Yvonne

Yvonnes are giant frogs that usually hide in rivers. They wait and jump out when someone passes by and attack.

Giant Frog Guardian

Early in the game after defeating Cow Bones you get another letter in the Suggestion Box telling about a giant frog Guardian. The Guardian seems to be a more grown up version of the enemies or maybe another species of UMA all together.

Attacks

Both Yvonne and the Guardian that either is one or just similar have similar attacks to. Both have a tongue lash that can instantly kill any one of your people and a lunge that makes a shock wave. The Guardian version also can jump off-screen and land causing a large shock wave. And it has a poison spit attack that splats on the screen.